Various inorganic or organic semiconductors have been used for manufacturing a solar cell, but only a silicon-based solar cell and a CIS-based or CIGS-based thin film (hereinafter referred to as “CI(G)S-based thin film”) solar cell have been commercialized.
Although a silicon-based solar cell has high photoconversion efficiency, its manufacturing cost is high. For this reason, there have been many attempts to manufacture a thin film solar cell using a compound semiconductor which can be shaped into a thin form.
Generally, such a thin film solar cell comprising a compound semiconductor is produced using elements of groups 11, 13 and 16 which typically involve the use of a CI(G)S thin film as the light absorption layer. In such a thin film solar cell, the light absorption layer, the buffer layer and the window layer are made of CuIn(Ga)Se2, CdS and n-type semiconductor, respectively. In particular, the light absorption layer of a solar cell determines the solar cell's capacity.
A CI(G)S light absorption layer is generally made by co-evaporation of appropriate metals under a gaseous Se atmosphere, or by selenization of a Cu—Ga—In alloy layer preformed using a evaporation deposition method using a gaseous selenium compound. A solar cell comprising a CI(G)S light absorption layer manufactured by the above methods shows a high photoconversion efficiency. However, these methods are not suitable for making a large scale thin layer and hampered by the problem of air pollution resulting from the use of toxic gaseous selenium or its compound, besides the additional problem of raw material loss caused by the use of vacuum equipments, which raises the manufacturing cost.
Low-cost manufacture processes of a thin film which do not required the use of vacuum equipments such as a chemical spray pyrolysis method and a paste coating process using metal precursors have been reported [U.S. Pat. No. 6,127,202 in the name of Kapur et al.; and U.S. Pat. No. 5,910,336 in the name of Ishihara et al.] However, these methods still suffer from the problem in that they use toxic H2Se or gaseous Se in the selenization step.
Thus, there exists a need to develop a low cost method of manufacturing a compound semiconductor for a large-scale film. For such a method, it is desirable to prepare the light absorption layer using a paste or an ink comprising a CI(G)S material.